The invention relates to cleaning processes for bipolar semiconductor manufacturing, and in particular to a pre-metal deposition cleaning process that produces bipolar transistors having low base-emitter voltage.
Base-emitter voltage, V.sub.be, is normally measured at 10 mA at the end of the front metal wet etch step as part of the wet etch process specification in a bipolar transistor manufacturing process. A wafer is scrapped if the V.sub.be is greater than 1.1 volt at 10 mA. It is well known that high V.sub.be is one of the major problems in bipolar device processing. The number of wafers scrapped due to high V.sub.be at the front metal wet etch step can be as much as 10% to 15%.
The invention provides a pre-metal deposition cleaning process for bipolar semiconductors that consistently produces wafers having low V.sub.be. According to the invention, the cleaning procedure includes a two step boron glass etching procedure: a chemical etchant consisting of DI (deionized water), H.sub.2 SO.sub.4, HNO.sub.3 and HCl (500:65:325:163) heated to 80 deg. C. followed by a 10:1 (DI:HF) dip. The semiconductor wafer is then annealed in forming gas. Then the boron glass etching procedure is repeated. The initial etch removes any B-Si-O glass present at base contacts. The anneal removes any fluorine clustered under the boron skin, and the final etch removes any retained B-Si-0-F phase from the anneal step. The cleaning procedure produces bipolar semiconductors with low V.sub.be. The procedure of the invention can also be used for rework of high V.sub.be wafers without the anneal and the second boron glass etch steps.